Groundwater modelling report - Lower Macquarie. CSIRO: Water for a Healthy Country National Research Flagship

[摘要] The existing groundwater flow model (Bilge, 2006) has been used to assist with the assessment of a preliminary extraction limit for groundwater pumping from the Lower Macquarie alluvial aquifer. To this end a total of seven fifty-year predictive model scenarios have been run and the results assessed against four criteria aimed at determining the sustainability of the particular scenario. These criteria are:• Criterion 1. Stabilisation of groundwater levels (productive base and key environmental outcome requirement).To meet this sustainability criterion groundwater levels must have stabilised or be rising at the completion of the scenario model run (i.e. after 43 years of extraction at the defined preliminary extraction limit) at the RCI (Resource Condition Indicator) sites.• Criterion 2. Ability to maintain volume of extraction (productive base requirement).The groundwater extraction rates are automatically reduced in groundwater models if a model cell from which groundwater is being extracted dries out due to excessive drawdown.To meet this criteria the model must maintain pumping at the required rate for the duration of the scenario model run. • Criterion 3. Prevention of dewatering of confined aquifers (productive base and key environmental outcome requirement).To meet this criterion the predicted groundwater levels at all RCI sites must remain above the top of a confined aquifer.• Criterion 4. Impact on surface-water streamflows (key environmental assets and key ecosystem functions requirement).This criterion mandates that the sustainable extraction limit must be equal to or less than the current level of groundwater extraction.In this regard the current level of groundwater extraction is defined as the average of the last five years of groundwater extraction.The current extraction limit for the Lower Macquarie alluvial aquifer is 75 GL/year. This pumping rate was used for Scenario 2 and it resulted in sustainability criteria being breached – groundwater levels do not stabilise and the confined aquifer is partially dewatered at one RCI site. The originally proposed preliminary extraction limit in the Lower Macquarie aquifer was 65 GL/year. This is equivalent to the current extraction limits in all management zones within the proposed sustainable diversion limit (SDL) management unit except for a 20 percent reduction in extraction from sub-zones one and six. The modelling results obtained when this level of extraction is assumed, suggest that it is not sustainable due to continued decline in predicted water levels and to dewatering of the confined aquifer. In fact all scenarios resulted in declines in groundwater extraction due to drawdown, drying of cells within the productive aquifers and associated loss of production capacity. It is concluded that the initially proposed extraction limit of 65 GL/year is not sustainable. As a result Scenario 8 was formulated and run with a reduced extraction limit of 35 GL/year. While this model provides relatively stable hydrographs at the RCI locations, extraction rates decline slightly below the target rate indicating desaturation of model cells that host groundwater extraction. The level of reduction is relatively minor. It was concluded that 35 GL/year is sustainable provided some of the extraction is redistributed to avoid drying of productive aquifer cells.The model covers all of the proposed SDL management unit and it was calibrated over the period from 1980 to 2002. It incorporates flood inundation and irrigation, but does not include evapotranspiration. A model uncertainty factor of 0.81 was adopted and the estimated preliminary extraction limit is 28 GL/year.

[发布机构] CSIRO

[学科分类] 地球科学(综合)